The first prototype of the mesoscopic family of optoelectronic devices was the dye-sensitized solar cell (DSC), which accomplishes optical absorption and charge separation by combining a light-absorbing material (the sensitizer) with a wide band gap semiconductor of nanocrystalline morphology. The DSC is used in conjunction with electrolytes, ionic liquids, polymer electrolytes or organic as well as inorganic semiconductors. Other strategies employ blends of organic materials, such as polymeric or molecular semiconductors as well as hybrid cells using a p-type semiconductor polymer in a conjunction with a fullerene or CdSe nanostructures [2]. The mesoscopic morphology of materials used in these PV devices is essential for their efficient operation, suffering by presenting low energy efficiencies, which is limited to the exposed absorber area. This is critical for the most common optoelectronic device able to convert light in energy, as it is the case of solar cells. This approach is not the one of the present invention.
Most solar cells industrially produced are currently based on silicon wafers. As a result, the price of photovoltaic modules is very high and it is still expected to increase due to a fast growing demand and a shortage in silicon production. The cost of the most efficient (30%) tandem solar cells based on III/IV semiconductors is very high, limiting their applications to space and solar concentrators [2]. Single-crystal silicon solar cells with power conversion efficiency of more than 25% are heavy and fragile, which prevents them from being used for mobile applications [3]. However, there is an increasing awareness of the possible advantages of devices based on mesoscopic inorganic and organic semiconductors commonly referred to bulk junction dye to their interconnected 3D structure. Mesoscopic injection solar cells operate in, an entirely different manner from conventional solar p-n junction devices. Mimicking the principles of natural photosynthesis, they separate light harvesting from charge-carrier transport. The semiconductors that conventional cells use assume both functions simultaneously, imposing stringent demands on purity and entailing high material and production costs.
Several concepts have been developed to tackle problems related to conventional solar cell technology. Inorganic thin film devices (based for example on CuInGaSe2) have reached efficiencies above 10.5% ready to enter to a commercial stage [4]. The evolution of dye-sensitized photoelectrochemical cell with inorganic semiconductor has continued progressively since 1991, reaching conversion efficiency of the single-junction cell currently at 12% [2]. Organic solar cells are on the other hand expected to show lower production costs and easier integration on flexible substrates. Yet they still have low efficiencies (around 5% at best) and suffer from short lifetimes as compared to inorganic devices.
Generally, to improve solar cell operation a number of physical processes must be optimized. These phenomena are common to almost all types of solar cells. First of all, when sunlight reaches the solar cell, some of the light is reflected back and lost. A polished Si surface reflects as much as ˜37% of light. Special antireflection layer is used to minimize the losses, reducing the reflection to ˜18%. In order to reduce the reflection, further, surface texturing is often used, which has shown to reduce the reflection to ˜13% [5]. Texturing of monocrystalline silicon is usually done by etching in alkaline solution, since multicrystalline silicon is textured usually by reaction with acid, by reactive ion etching or by mechanical and laser processing [6]. Improvement in performance of textured solar cells is significant. Unfortunately, those methods are first, suitable only for silicon technology; secondly, it's very difficult to incorporate those solutions in production lines.
As far as mesoscopic based solar cells are concerned such as DSC, the active mesoscopic semiconductor layer needs to be thick and porous enough to harvest as many photons as possible. In case of mesoscopic cells, the mesoporous semiconductor film is critical to device performance. A 10 μm thick film increases the total internal surface area by a potential 1000-fold over the geometric smooth surface to maximize light harvesting [2]. Moreover, photo-generation, separation and recombination take place nearly exclusively on the surface of the nanoparticles, and thus the properties of the interface is of critical importance to the conversion efficiency of the device. The mesoporous semiconductor layer enhances the effective surface area for sensitizer molecule attachment for light absorption. In the same time, DSC with varying thicknesses of active layer (for instance, ranging from 3.9 to 12 μm) show qualitatively the same behavior, which shows that the semiconductor layer thickness has no critical influence on peak position [7,8].
And last but not least, the electrons or/and holes created by the absorbed photons must be separated and collected to generate electricity with minimum loss. The Complete equation for the incident photon to current conversion efficiency (IPCE) is the following:
                    IPCE        ≅                                            1240              ×                                                J                  sc                                ⁡                                  (                  λ                  )                                                                    λ              ⁢                                                          ⁢                              W                i                                              ×                                    η              ×              τ                                      t              i                                ×          1          ⁢                                              (        1        )            where Jsc is the short-circuit current density (μA cm−2); λ the excitation wavelength (nm); Wi is the photon flux (W m−2); η describes the efficiency of the generation charge process (number of electron-hole pairs produced per absorbed photon); τ/tt is the ratio between the recombination life time and the free carriers' (electrons) transit time (usually (η×τ)/tt≈1); 1 represents the photons absorbed by the light sensitive layer, that depends on the product between the absorption coefficient (α) and the layers' thickness (d). For αd>0.4 (high absorption region), the terms between brackets is approximately equal to one, while for αd≦0.4 (weakly absorbed region), the term between brackets is approximately equal to αd, leading to a linear dependence of IPCE on αd. Conducted experiments shows that there is a linear growth of IPCE with thickness (up to around 10 μm) in DSC, in the so-called low absorption regime [7,9]. However, for thicker layers (>10 μm), a very slow decrease of photocurrent is observed because the average path length for the electron diffusion becomes longer and the recombination probability increases. The TiO2 nanocrystals do not have to be electronically doped to render them conducting because the injection of one electron from the sensitizer into a 20 nm sized TiO2 particle suffices to switch the latter from an insulating to a conductive state. This photo-induced conductivity of the particle films allows collecting the electrons without significant ohmic loss [2]. Although some of the physical principles of the present invention aims similar objectives, the method and approached are different from the ones described in the literature.
Nanostructured surfaces like for example nanowires for mesoscopic solar cell applications have been studied by several research groups, and there also been a strong interest to explore these nanostructures for antireflection and light-trapping applications [10,11]. Solar cells comprising nanostructured surfaces as an active element have been also described in U.S. pending patent applications. The U.S. patent application submitted by Zhang et al. (2007/0111368A1, May 2007) describes a photovoltaic structure along with method for forming PV structure with a conductive nanowire array electrode. The nanowires are covered with two semiconductor layers. The first semiconductor layer includes n-type dopant while the second semiconductive layer includes a p-type dopant. The increase in active surface area thanks to the application of conductive nanowire array electrode is based on different principles that those postulated in present invention. The mechanism of light harvesting and manufacturing technology is also quite different from the present innovation.
Recently, Kelzenberg et al. designed a silicon microwire-array structure that improves the efficiency of all main processes in solar cell [12]. Based on silicon CVD-grown microwires, a solar cell which has achieved efficient absorption of sunlight while using only 1% of the active material used in conventional designs has been developed. Results of this work show that microwire arrays are very efficient not only in the absorption of sunlight but also in the collection of charge carriers. This is out of the scope of the present invention, concerning method and final devices (not micropillars but nanowires, a fully different approach).
The U.S. patent application submitted by Korevaar et. al. (2008/0047604A1, February 2008) is directed to photovoltaic devices comprising Si nanowires as active elements in p-i-n type cell. By providing crystalline Si nanowires in an amorphous Si matrix, the path of the holes is dramatically reduced, thereby increasing, hole collection and corresponding device efficiency. This way of efficiency enhancement is different than the mechanism postulated in the present invention.
Attempts to prepare molecular-based solar cells have generally employed molecules in conjunction with semiconducting films (often TiO2) or nanorods (such as CdSe). Conventional molecular-based cells employ several designs. For example, a monolayer of pigment can be deposited on semiconductor surface. A monolayer of pigment can be also bounded to a mesoporous semiconductor layer. In each of these designs, the absorption of light by the pigment can results in electron injection into the semiconductor. However, in practice, conventional molecular-based cellular designs are limited in their efficiency, limitation that it is overcome by the present invention.
A way of smoothly carrying out transfer of electrons and having high photoelectric conversion efficiency but applied in dye-sensitized solar cells have been described in U.S. patent document submitted by Den et al. (U.S. Pat. No. 7,087,831B2, August 2006). The absorption layer-modified semiconductor mixed crystal layer comprises a semiconductor acicular crystal and, a light absorption layer formed on the surface of the crystal. The semiconductor acicular crystal is used as one charge transfer layer and, the light absorption layer is formed between the charge transfer layer (working electrode) and the top charge transfer layer (counter electrode). The method based on the same principles but using different materials and deposition technique have been described in U.S. patent document by Yang et al. (U.S. Pat. No. 7,265,037 B2, September 2007). The main embodiment of these inventions are directed to a method of, deposition ZnO nanocrystals on the substrate using dip coating process and containing the substrate with solution of zinc nitrate hexahydrate and methenamine. Application of acicular semiconductive crystals described in both presented above patents represents a different approach than described in present invention which is more similar to the classical approach postulated by Gratzel [2].
Other methods for nanostructure production and their deposition on the substrate have been described in other U.S. patent documents submitted by Romano et al. (U.S. Pat. No. 7,339,184B2, March 2008; and U.S. Pat. No. 7,344,961B3, March 2008) and Duan et al. (2006/0211183A1, September 2006), which are out of the scope of the present invention. Photovoltaic devices based on nanowire array approach have been also presented in U.S. patent applications by Parsons (2007/0025139A1, February 2007) and Wang et al. (2009/0189145A1, July 2009), whose one of the applications converge to the one of the present patent but the method the structure and compositions are absolutely different.
As a generic comment, the above described devices are essentially based on nanowires and nanorod arrays, some aiming to be used in sensing or photovoltaic applications, fully different of the method and device concept behind the present invention. Both of these proposed device solutions should not be confused with each other as they do not have much in common. The main difference is that the present invention describes a different method of micro- or nano-structures development, completely out of the scope of the previous mentioned devices and processes that represent as far as we know the present state of the art of science and technology.
Performance improvement of all mesoscopic devices is so far the way to increase photocurrent densities using new sensitizer without changing the currently used stable ion conductors. One particular weakness in present mesoscopic solar cells is the presence of a massive number of interfacial boundaries acting as trap sites for electron transport. Another flaw that works against the notion of large metal oxide surface area is the presence of interfaces exposed to the ion conductor. These exposed surfaces, which are not anchored with sensitizer molecules would be in direct contact with ion conductor and provide new recombination pathways as electrons are lost to the redox couple when percolating through the network.
An additional problem in the today's state of the art in the use of mesoscopic devices in many applications is its inconvenient planar structure. The obvious solution to this problem could be application of lateral structure what appears to be only apparently easy [13]. Interdigital electrodes made up of two individually addressable interdigital comb-like electrode structures have frequently been suggested in application of mesoscopic PV devices (Wang et al., U.S. Patent Application Publication, 2009/0188552A1, July 2009). However, its use was limited only to electrical signal collection in large area PV modules, out of the scope of the present invention.